In Active Matrix Organic Light Emitting Diode (AMOLED) displays, a voltage data signal is converted in the pixel to an OLED driving current (and thus brightness) by a thin film transistor (TFT). Conventional “bottom-anode (ITO)” OLEDs typically use p-channel devices, because only p-channel devices allow for the direct programming of the TFT gate-source voltage (and thus the driving current) independent of the OLED I-V curve. Accordingly, low-cost amorphous-Si (a-Si) and amorphous oxide semiconductor TFT technologies cannot currently be used in active matrix displays because no p-channel a-Si or amorphous oxide semiconductor device exists for fundamental reasons, which will be described in greater detail below. Therefore, more expensive materials, such as low temperature poly-Si (LTPS) must be used to form active matrix display devices.
Oxide semiconductor-based TFTs have attracted an increasing amount of attention due to their desirable characteristics, including high mobility, low-temperature process compatibility, and low fabrication cost. However, in oxide semiconductor-based TFTs (e.g., amorphous indium-gallium-zinc oxide (a-IGZO), indium-zinc-oxide (IZO), gallium-zinc-oxide (GaZnO), aluminum-zinc-oxide (AlZnO), zinc-oxide (ZnO), copper-oxide (Cu2O), tin-oxide (SnO), nickel-oxide (NiO), copper-aluminum oxide (CuAl2O4), zinc-rhodium-oxide (ZnRh2O4), or the like), the electrical performance of p-type oxide TFTs is still inferior to that of n-type oxide TFTs. Specifically, it has been found that the electrical performance of p-type oxide-semiconductors is severely limited by the localized 2p orbitals in the valence band maximum (VBM), the deep VBMs, and self-compensation by donor impurities. Accordingly, it is currently not feasible to use oxide semiconductor-based TFTs in AMOLED devices or other display technologies, such as Active Matrix Inorganic Light Emitting Diodes, micro LED Displays, or the like.
Accordingly, there is a need for p-channel oxide semiconductor thin film transistors.